The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 on which is antiferromagnetic layer 12 whose purpose is to act as a pinning layer for magnetically pinned layer 345. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1). Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A contacting layer 18 lies atop free layer 17 and cap layer 19 is present over layer 18 to protect the structure during processing.
When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8–20%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film's plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. By contrast, in a CPP device, the resistivity of the leads tend to dominate and should be as low as possible.
For application of the CPP SV structure in a reader head, some developers have substituted a CoFe_Cu layer for pure CoFe in both the free layer and in AP1. CoFe_Cu is shorthand for a CoFe layer within which has been inserted a very thin laminate of pure copper (designated as 20 in FIG. 1). It has been found that the substitution of CoFe_Cu for CoFe, or a CoFe/NiFe composite, leads to higher values of CPP GMR and DRA. While this represents an important improvement in device behavior, it has been found that this type of structure exhibits very negative magnetostriction, in the range of high −10−6 to −10−5.
Magnetostriction is an important form of anisotropy in magnetic materials. It relates the stress in a magnetic material to an anisotropy created by that stress. The dimensionless magnetostriction constant λ is a constant of proportionality. If λ is positive, then application of a tensile (stretching) stress to a bar will create an easy axis in the direction of the applied stress. If a compressive stress is applied, then the direction of the easy axis created will be perpendicular to the stress direction. If the magnetostriction constant for the material is negative, then this is reversed.
The magnetostriction constant λ is defined as follows: If a rod having a positive value of λ is magnetized, the rod will stretch in the direction of the magnetization. The fractional increase in length defines the magnetostriction constant λ. If the material has a negative value of λ, then the material will shorten in the direction of the magnetization. The latter is an undesirable value of the magnetostriction for head operation because in actual read heads the stress is compressive. The resulting very large negative magnetostriction will generate large anisotropy along the stripe length direction which greatly reduces the heads output. For read head applications, a low positive magnetostriction is preferred.
The present invention discloses a method to adjust the free layer magnetostriction to a very low positive value.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,627,704, Lederman et al. show a GMR CPP transducer with a flux guide yoke structure but no details are given about the makeup of the stack. In U.S. Pat. No. 6,118,624, Fukuzawa et al. describe a layer of NiFe or the like between the pinned layer and the free layer and adding an oxygen layer between the NiFe and the pinned layer. Dykes et al., in U.S. Pat. No. 5,668,688, show a CCP GMR structure, but give no details about the makeup of the structure.